1. Field of the Invention
The present invention relates generally to mark detectors, drive controllers, belt drive units, and image forming apparatuses, and more particularly to a mark detector for appropriately rotating an endless belt member, a drive controller including the same, a belt drive unit including the drive controller and the endless belt member, and an image forming apparatus, such as a copier, a printer, or a facsimile machine, including the belt drive unit.
2. Description of the Related Art
Some electrophotographic image forming apparatuses have multiple primary transfer parts (primary transfer means) to successively transfer respective single-color images formed with corresponding single-color toners on corresponding multiple photosensitive bodies (first image carriers) onto an intermediate transfer body (second image carrier), thereby superposing the single-color images one over another so as to form a composite color image; and a secondary transfer part (secondary transfer means) to transfer the composite color image formed on the intermediate transfer body onto a sheet of paper. Other electrophotographic image forming apparatuses have a primary transfer part to successively transfer single-color images formed successively with corresponding single-color toners on a photosensitive body onto an intermediate transfer body, thereby superposing the single-color images one over another so as to form a composite color image; and a secondary transfer part to transfer the composite color image formed on the intermediate transfer body onto a sheet of paper.
In such image forming apparatuses, for example, those having endless belt members for image formation, such as a belt-like photosensitive body (photosensitive body belt), a belt-like intermediate transfer body (intermediate transfer belt), and a paper conveyor belt, it is required to control the amount of movement and the movement position of the endless belt member (actually, its moving surface) with accuracy in order to accurately position the endless belt member and an image (toner image) on a sheet of paper (transfer material) conveyed by the endless belt member.
However, the movement velocity of the endless belt member is likely to vary because of various factors such as load variations caused by a member contacting the endless belt member. Accordingly, it is extremely difficult to eliminate variations in the velocity of the endless belt member completely. Therefore, if the endless belt member is caused to vary for some reasons, its movement velocity, amount of movement, and movement position also vary. This results in a problem in that it is difficult to control error in the positions of the endless belt member and an image on a sheet of paper conveyed by the endless belt member with high accuracy.
In order to eliminate this disadvantage, an image forming apparatus is proposed in which: a rotary encoder is directly coupled to the rotary shaft of an endless drum-like member or the rotary shaft of a driving roller (for moving an endless belt member) in order to control error in the position of an image due to variations in the rotational angular velocity of the endless drum-like member or the driving roller with high accuracy; and the rotational angular velocity of a drive motor serving as means to drive the endless drum-like member or the driving roller is controlled based on the rotational angular velocity of the endless drum-like member or the driving roller detected by the rotary encoder (see, for example, Japanese Patent No. 3107259). This image forming apparatus indirectly controls the amount of movement (movement position) of the endless drum-like member or the endless belt member by controlling the rotational angular velocity of the endless drum-like member or the driving roller.
On the other hand, an image forming apparatus is proposed in which: marks (or holes) are formed on the exterior surface (top surface) or the interior surface (bottom surface) of a belt (endless belt member) so as to be successive at predetermined intervals along the moving direction of the belt; and the movement velocity of the belt surface is calculated from a pulse interval obtained by detecting the marks with a sensor (mark sensor) and is fed back to a drive control (see, for example, Japanese Laid-Open Patent Application Nos. 6-263281, 9-114348, and 11-24507). According to this image forming apparatus, it is possible to directly observe the behavior of the belt surface. Accordingly, it is possible to directly control its amount of movement. As a result, it is possible to reduce the eccentricity of a driving roller for driving the belt, skidding between the driving roller and the belt, and detection error due to the thickness deviation of the belt, thus making it possible to perform drive control with high accuracy.
In general, however, it is extremely difficult to have an endless belt member formed to be uniform in thickness in its direction of movement (rotational direction). Further, the thickness of the endless belt member varies because of deformation of the endless belt member due to tension applied thereto during its movement. Therefore, while the endless belt member is moving, a change is caused in the distance between the endless belt member (marks) and a mark sensor. Further, in the case of detecting marks in a part of the endless belt member (belt part) between multiple support members supporting the endless belt member, a change is also caused in the distance between the endless belt member and the mark sensor by the vibration of the belt part. Further, an angle (angular error) greater than a prescribed range may be formed at the time of attaching the mark sensor.
Therefore, in the case of optically detecting multiple marks on the endless belt member, that is, in the case of detecting multiple marks on the endless belt member using a light-reflection-type mark sensor having light emitting means to emit a beam (light beam) onto the light illumination surface (marks) of the endless belt member and light receiving means to receive reflected light from the light illumination surface, if a change in the distance between the mark sensor and the light illumination surface (detection distance) goes beyond a prescribed range because of the above-described thickness or vibration of the endless belt member or the attachment angle goes beyond a prescribed range at the time of attaching the mark sensor, the angle formed between the light illumination surface and the optical axis of the beam emitted from the light emitting means onto the light illumination surface goes beyond a range that does not affect image quality. This causes a problem in that timing error occurs in mark detection so as to cause detection error.